it
r.
Floral and Attendant Faunal Changes on the Lower
Rio Grande Between Fort Quitman and Presidio, Texas 1
I­ Ll Ronald W. Engel-Wilson and Robert D. Ohmart
2
Abstract.--Written and photographic documentation from
explorers and settlers demonstrate that the floodplain was
historically dominated by cottonwood-willow (Populus fremontii­
Salix gooddingii) and screwbean mesquite (Prosopis pubescens)
communities. Past and present land and water use practices
have resulted in an almost complete elimination of native
tree species and a dominance by the exotic salt cedar
(Tamarix chinensis). Avian census data from the area show
higher bird population densities and diversites in the
select mature cottonwood-willow communities than in mature
salt cedar communities.
INTRODUCTION
This paper examines historical records
containing vegetation descriptions for the
Rio Grande from Presidio to Fort Quitman,
Texas, and compares past vegetation with
current vegetative conditions. The bird
species in existing plant communities are
examined, and changes in avifaunal diversities
and densities since European settlement are
evaluated based on our premise that as plant
communities change through time so do the
attendant bird species.
The correlation of riparian habitats and
high avifaunal densities and diversities has
been documented numerous times (Carothers
1977, Hubbard 1977, Anderson et al. 1977b,
Stevens et al. 1977, and Wauer-1977) •
Andersonet-;1. (1977b) found that avian use
of the exoti~salt cedar (Tamarix chinensis)
on the Colorado River was lower than avian use
of native riparian habitats, and this general
lPaper presented at the National
bymposium on Strategies for Protection and
Management of Floodplain Wetlands and other
Riparian Ecosystems, Pine Mountain, Georgia,
December 11-13, 1978.
,
.j.
2Respectively, Graduate Student, and
Associate Professor of Zoology, Arizona
State University, Department of Zoology
and Center for Environmental Studies,
Tempe, Arizona 85281.
trend has also been observed on the Rio Grande.
More avian habitat specialists were found in
cottonwood-willow (Populus fremontii-Salix
spp. ) .·.and screwbean mesquite (Prosopis
pubescens) communities during the winter than
any other community type; yet in aerial extent
these two communities covered less than 40 ha
within a 20,000 ha study area (Engel-Wilson
and Ohmart 1978). This is highly important
in view of the decreasing area of native
riparian habitat and its replacement with
agriculture or salt cedar (Robinson 1965,
Harris 1966, and Horton 1977).
Historically the trade route along the
Rio Grande from Presidio to Fort Quitman,
Texas, was traveled infrequently due to the
rugged terrain and hostile Indians. The
narrow and densely vegetated floodplain
paralleled by the rugged upland terrain made
travel difficult and frequently impossible,
so few travelers visited the area. Consequently
early accounts are scarce, but they do describe
the nature of the riparian plant communities
that occupied this floodplain.
Spaniards were the first Europeans to
establish settlements on this section of the
Rio Grande. Forts and missions were estab­
lished at Presidio as early as 1609 (Bancroft
1886) and at El Paso in 1683 (Espinosa 1940).
In 1582 an early Spanish explorer, Espejo
(Hammond 1929), wrote of the extent of the
This research was supported by International
Boundary and Water Commission grant IBM 77-17.
139 Saturday, March 3l •.. 0n our left on the
opposite bank is the North end of the
Sierra Grande [,Sierra Alto]. On our
right [is] a high and large mountain which
I have called "Mount Barnard" [Chinati
Peak]. The travel has been much inter­
rupted by chaparral, the cactus in every
variety, enormous palmitios from fifteen
to twenty-five feet high, and every kind
of thorn.
vegetation and the plant communities of the
Rio Grande just above San Bernardino (now
Presidio, Texas). He stated:
It is such a river that all of these
three leagues [in width] [1 league=:4 km]
are covered with numerous groves of
poplars and willows there being in it
very few willows or any other sort
of trees.
In 1846 Whiting (Bieber 1935) wrote of the
vegetation he saw along the river during his
march to establish a wagon route from San
Antonio to El Paso as he followed the Rio
Grande from Presidio northward (Fig. 1).
Uni ted
Monday, April Z•.• On account of the under­
growth and the willow, our progress was
laborious and slow, and in many places
we were forced to the hills ••• the Rio
Grande with its pale green fringe of
cottonwoods and willows, formed a pleasant
picture. Towards dusk we reached a bottom
where though the grass was poor, we were
obliged to camp.
Thursday, April 5 .•• we saw a beautiful
valley with its usual clothing of cotton­
wood, stretching to the northward •.. To our
right now lies .•. a rugged and high
mountain ••. this I called "Mount Chase."
[Eagle Mountain]. Monday, April 9 •..
continued the march in one unbroken valley,
through broad and level bottoms heavily
timbered with large cottonwoods ... we saw
a flock of twenty-five huge white pelicans
.•. Three large,black-tailed deer, the
mountain species were wounded ..• At four the
march was resurned ••. still in the extensive
groves of alamos [Cottonwoods], becoming
wider as we advance. In this tract the
grass is generally poor •..
States
N
Mexico
• Chlnat I P•• II
S Ie'" AIIO'
MI.
',5
Tuesday, April lO •.• Our trail still lay
in the broad cottonwood bottom. The soil,
nearly destitute of grass, ... Wednesday,
April 11 .•• This trail lies through a fine
tract, heavily timbered--the trees are
very large. We saw nothing of the southern
chaparral and undergrowth to impede us ...
We are now opposite to the island called
"La Isla" ••. [near San Elizario] a mexican
visited our camp ••• told us we were thirty
miles from El Paso.
3p
Figure I.Map of the Rio Grande from El Paso
to Presidio, Texas.
He viewed the floodplain from a hill over­
looking Presidio and Ojinaga, Chihuahua and
wrote:
Saturday, March Z4 ••• We soon carne in
view of the Rio Grande with its green
valley and cottonwood groves. Friday,
March 30 •.. Nearly opposite the Mexican
town we passed a fine part of the valley
where the mesquite, the willow and cotton­
wood abound in size, and where some
judicious and tasteful clearing would make
a pleasant site for quarters of a post, •••
Our trail ..• between cottonwoods, which
almost everwhere line the banks of the
river, and the gravel ridges which put out
from the hills or rather mountains
[Chinati Mountains] on the right.
The flora of this section of the Rio
Grande Valley was described by Dr. Parry who
accompanied the United States and Mexican
boundary survey in 1857.
In the valley of the Rio Grande we
frequently find a heavy growth of cotton
wood and willows. The "screw bean,"
Strombocarpa pubescens, often occupies
large tracts, accompanied by a dense
undergrowth of Baccharis salicina. The low
saline places produce an abundance of
~ canescens, while on the higher
140
ground Tessaria borealis is a common
plant. (Emory 1859).
probably le~s than 800 ha under production,
the remaining being fallow or in pasture.
The early descriptions of Espejo and
Whiting indicate there was little change in
the vegetation along the Rio Grande between
1582 and 1846. Farming and livestock
operations were initiated by the Spanish in
the late 1500's and reached their zenith in
the late 1800's and early 1900's. By the
early 1900's much of the bottomland vegetation,
mostly cottonwoods, had been removed to
develop the riparian floodplain for agricul­
tural purposes (Horton 1977), building material
(Emory 1859), livestock forage (Galvin 1966),
and fuel.
Due to the decrease in agricultural
activity, the floodplain was left open for
the invasion of the exotic salt cedar (Harris
1966, and Robinson 1965) (Plate 2). Cotton­
woods and willows were unable to grow in
their former areas because 1) the abandoned
soils were not suitable, 2) the water quality
and availability were inadequate, and 3) annual
floods had been stopped thereby eliminating
soil suitable for seed propagation. The very
nature of salt cedar growth along intermittent
river courses helps salt cedar disperse into
low-lying farmland. Intermittent flows allow
seedlings to invade the channel during reduced
flows, and when the water is high the seedlings
trap the sediment. Annual repetition of this
process eventually results in soil and
vegetation blocks in the channel, which causes
channel shifts and flooding of adjacent areas
(Robinson 1965, and Harris 1966).
Photographs of the area in 1918-1919 show
a dramatic change in the vegetation of the
alluvial floodplain from that described by
Whiting in 1849. The only remaining cotton­
woods and willows wer.e those along the river
and along the margins of the fields (Plate 1).
I
I
An assessment of the plant communities
in the study area, from March 1977 to March
1978, demonstrated there has been a drastic
change in species composition and aerial
extent of the native plant communities during
the past century. The degree of clearing in
the valley was determined by comparing hectares
under cultivation in the early 1900's to the
present time. The total area under cultivation
in 1928-1930 was 7,280 ha (IBWC 1978), and by
1977 this had dropped to 4,700 ha (Engel-Wilson
and Ohmart 1977). Peak values for Mexico were
6,228 ha in 1949 (IBWC 1978). The total
cleared may be somewhat less than the sum
of the two extremes due .to boundary shifts.
shifts.
Plate 1. Candelaria 1918. Looking down
river one can see numerous large cottonwoods
and much agricultural activity.
The s1!bsequent decline in agriculture
appears to be due to several factors and/or
a combination there of:
1. Increased use of water upstream
2. Construction of dams upstream
(Elephant Butte 1919 and Caballo
1938)
3. Economic depression of the 1930's
4. Floods of 1942 and 1966
5. Increased salt accumulations in
floodplain soils which reduced
agricultural productivity
Although there are 4,800 ha (23 percent of the
floodplain) of recognized agricultural land
in the valley, it is noteworthy that there are
Plate 2. Candelaria 1974. Most of the
vegetation is salt cedar with little
riparian vegetation.
141
In the study area in 1977 there were
5,600 ha of salt cedar which comprised about
27 percent of the floodplain vegetation
(Engel-Wilson and Ohmart 1977).
Only about
12 ha of mature cottonwood and willow persisted
which made up only 0.06 percent of the flood­
plain. This did not include isolated indiv­
iduals along canals and fields. Plates
1 and 2 contrast the abundance of cottonwood
in 1918 versus 1974. The remaining flood­
plain was made up of a mixture of honey
mesquite (Prosopis velutina) and other desert
shrubs (9600 ha or 46 percent).
association of trees, shrubs and forbs. Percent ground cover for the most common plants are: cottonwood 5 percent, willow 14 percent, salt cedar 7 percent,seepwillow (Baccharis spp.) 17 percent, Johnsongrass (Sorghum halepense) 34 percent and dock (Rumex spp.) 32 percent. -----The relative foliage volume (RFV) in the
salt cedar community was 2.848 m2 /m 3 with
0.256 m2 /m 3 RFV above 7.5 m with no vegetation
above 9 m. The RFV in the cottonwood-willow
community was 6.208 m2 /m S with 0.499 m2 /m 3
RFV above 7.5 m and having vegetative volume
occuring up to 12 m.
Methods
Birds
A survey of historical literature and
photographs was conducted in the libraries
of Arizona State University, University of
Texas at El Paso, and El Paso City.
Transects were established in nine
mature salt cedar communities comprising a
total of 150 ha. One transect of 10 ha was
established in a cottonwood-willow association.
These areas were sampled for bird densities
each month for one complete year using the
Emlen (1971) technique (Anderson, etal. 1977a)
Densities were expressed as the numbe;-of birds
per 40 ha and presented as monthly means for
each of five seasonal groups: March-April
(MA); May through July (MJJ); August-September
(AS); October-November (ON); and December
through February (DJF). Species which
occurred in fewer than two seasonal groups for
both community types were not used in the
comparisons. Also ducks, herons and rap tors
were excluded from the analysis.
Foliage Height Diversity (FHD) and
relative foliage volume (RFV) were measured
in each community using the board technique
(MacArthur and MacArthur 1961).
Percent ground cover was estimated using
a 5 m2 area. Percent ground cover by plant
species was estimated to the nearest 5 percent
in the 5 m2 plots. Plots were located 5 m to
the right and left of the transect on 61 m
spacing beginning at 15 m from the beginning
of the transect. The sums of the percentage
for each plant species for aztransect was
divided by the number of 5-m 's to derive
the mean percentage of each plant species for
the transect.
RESULTS
Vegetation
Salt cedar forms a dense monotypic stand
with 91 percent ground cover and only traces
of annuals, forbs and other shrubs. The
cottonwood-willow community is a multi-layered
142
The highest number of species occurred
in salt cedar during March-April (see Table 1).
The lowest numbers of species (17) were
detected in the cottonwood-willow association
(Table 1) in these same two months. In par~
the latter may be an artifact of the small
number of samples from the cottonwood-willow
community at the beginning
the study.
The number of species recorded in low densities
(i.e. < 1/40 ha) in salt cedar was 10 and one in cottonwood-willow in MA. If very low density species are ignored, the number of species seen in salt cedar versus cottonwood­ willow during MA is reduced to 16:20. In the remaining seasons, little fluctuation in the number of species in the cottonwood-willow community was observed (27 May through July, 26 August through September, 27 October through November, and 23 December, January, and February). 0;
Densities were low for both communities in MA (475 CW, 330 SC), but increased to a peak in SC in MJJ (486) and to an AS peak (749) in CWo Both communities had decreased
densities in ON 06 SC, 569 CW) and densities
in SC continued to decline in DJF (67). How­
ever avian densities in CW increased to
744/40 ha in DJF due primarily to the influx
of granivores and small insectivores.
Differences in the bird populations of
these two communities became more striking
when a large portion of the dove population was
removed from the consideration. Doves gain
only a small fraction of their total food
intake in the communities in which they nest,
consequently we feel a reduction of 90 percent
of the doves is ecologically valid. The
difference in total birds with this adjustment
is dramatic between cottonwood-willow and salt
cedar in MA (418 to 181) MJJ (598 to 262) and ON
- (657 t6 80), respectively. Salt cedar appears
to be very good nesting habitat for White­
winged Doves as they reached nesting densities
in the Candelaria area of 28 nests per season
Table 1.
Bird densities (1/40 hal by month group for cottonwood-willow (CW) and salt cedar (se) communities
on the Rio Grande at Fort Quitman.
MA'
cw
I
Gambel Quail
(Lophortyr gambeliil
A-S'
MJJ'
cw
SC
II
I
SC
II
13
23
cw
I
DJF'
O-N'
SC
II
CW
I
SC
II
35
81
CW
I
SC
II
58
0
Mourning Dove
(Zenaida macroura)
9
17
16
28
27
9
15
o
o
0
White-winged Dove
(Zenaida asiatica)
53
149
107
221
76
100
0
o
o
0
o
o
016
o
1
600
o
o
0
10
1
o
o
o
0
Roadrunner
(Geococcyx calfgonianus)
o
Yellow-billed Cuckoo
o
0
38
33
Ladder-backed Woodpecker
(Picoides scalaris)
6
1
Common Flicker
(Calaptes auratus)
o
Ash-throated Flycatcher
(Hylarchus cinerascens)
o
Say Phoebe
(Sayornis
3
o
o
o
(COCCYZUS
o
a1'llericanus)
Black-chinned Hummingbird
(Archilocus alexandri)
19
6
3
o
11
o
o
o
o
1
o
2
o
o
o
o
o
o
o
o
11
o
0
o
1
~)
Black Phoebe
(Sayornia nigricans)
Vermilion Flycatcher
(pyrocephalus rubinus)
40
11
6
o
4
0
o
o
0
o
o
4
o
29
4
o
Verdin
(Auriparus flariceps)
8
1
o
2
18
4
29
o
o
o
o
o
6
o
o
o
o
o
0
o
o
o
10
17
13
8
o
o
o
18
6
0
50
17
93
6
o
o
0
o
o
o
0
1
o
0
237
0
3
13
26
Black-taile~'
o
o
o
5
o
Ye110w-rumped Warbler
(Dendroica coronata)
o
130
o
20
o
Bell Vireo
(Vireo bellii)
Lucy Warbler
(Vermivora luciae)
1
o
Cliff Swallow
(Petrochelidon pyrrhonota)
Gnatcatcher
(Polioptlla melanura)
12
0
Orange-crowned Warbler
(Vermivora ce1ata)
o
Yellowthroat
(Geoth1ypis trichas)
o
Yellow-breasted Chat
(~ virens)
o
17
94
55
54
11
Swmner Tanager
(Piranga !.!!l!!!!.)
o
16
29
29
32
11
80
10
116
16
50
4
12
o
o
o
o
41
o
42
0
o
o
o
o
17
1
6
0
HOuse Finch
(Carpodacus mexicanus)
Brown Towhee
o
(Pipilo fuscus)
Green-tailed Towhee
(Pipilo ch10rurus)
Cardinal
(Cardinali. cardinalis)
Painted Bunting
(Passerina ciria)
o
o
o
8
o
o
143 o
o
38
10
32
2
o
31
o
o
0
Table 1 (continued)
MJJ'
HA'
cw
cw
SC
II
I
II
cw
DJF 5
o-N'
A-S'
SC
SC
CW
SC
CW
o
18
SC
II
II
II
o
o
1
o
o
o
Long-billed Harsh Wren
(Cistothorus palustris)
o
o
o
o
o
o
11
o
13
0
Bewick Wren
(Thryo....nes bewickii)
2
3
1
2
o
3
38
8
22
9
Mockingbird
(M1mus polyglottos)
3
4
6
6
1
0
o
o
0
Crisssl Thrasher
(Toxostoma dorsale)
o
o
o
1
12
Hemi t Thrush
o
o
o
o
o
o
Northern Oriole
(~ galbula)
o
o
2
1
o
o
o
o
0
orchard Oriole
(~ spurius)
o
1
48
8
28
o
o
o
0
Red-winged Blackbird
(Agelaius phoeniceus)
1
14
11
5
6
o
o
o
o
0
Great-tailed Grackle
(Quiscalus mexicanus)
o
9
11
15
52
o
3
o
o
0
Brown-headed Cowbird
(Molothrus ater)
o
53
28
2
o
o
o
o
0
Loggerhead Shrikp
(Lanius ludovicianus)
o
o
o
o
2
1
o
o
0
12
4
o
o
o
o
41
24
64
32
o
o
29
4
56
o
o
o
0
o
o
25
6
40
1
o
0
White-crowned Sparrow
(Zonotrichia leucophrys)
35
o
1
o
o
o
139
1
108
5
White-throated Sparrow
(Zonotrichia albicollis)
o
5
o
1
o
o
o
o
o
0
29
o
o
o
o
o
58
o
2
o
o
o
o
o
o
17
o
31
0
o
o
o
o
o
o
12
o
43
0
House Wren
(Troglodytes aedon)
(~
o
1
o
guttatus)
Ruby-crowned Kinglet
(!!&ullls calendula)
Blue Grosbeak
(~ caerulea)
Lesser Goldfinch
(carduelis psaltda)
Chipping Sparrow
(Sp1:<ella passer ina)
Song Sparrow (Melospiza melodis) Swamp Sparrow (Melospi za georgiana) 17
30
27
28
26
25
27
20
23
13
475
330
708
486
749
178
569
76
744
67
419
181
80
556
76
744
67
Diverei ty All Bird.
598
2.670
657
2.243 2.248
2.160
2.8401.9262.7502.2672.3811.795
% of Max
0.792 0.660
0.810
0.648
0.8720.5980.8340.757 0.759 0.700
Diversity 10% Doves
2.124 2.879
2.659
2.737
2.7902.8532.7142.267 2.3811.795
% of Max
1 March to April
0.821
0.855 0.886 0.823 0.757 0.759 0.700
It
0.804
0.750 0.846
Oc taber to November
5
December, January, and February
N..... ered Species
Dens ity All Birds
'1
40 ha
Denisty 10% Doves
, Hay, June. and July
, August to Septe1llber
144 262
important has been the invasion of the exotic
salt cedar. Salt cedar has the ability to
colonize areas after summer rains because
its flowering and fruiting cycles are such
that there is a continual supply of seeds to
germinate when the soil is moist (Harris
1966). Once a dense carpet of salt cedar
seedlings has developed little light penetrates
to the soil for germinating cottonwoods or
willows. Should the latter germinate and
survive, the future of cottonwoods is bleak,
for in a few years the debris laden community
burns. Fire encourages the dominance of salt
cedar by removing other species which are less
fire adaptive (Ohmart, Deason, Burke 1977).
Salt accumulations (discussed by Ohmart et al.
1977) may also have played an important role­
in the elimination of cottonwoods as sediments
clogged the channel and spread salt-laden
waters laterally.
per 0.4 ha and population densities
of 506 per 40 ha (Engel-Wilson and Ohmart
1978).
Bird species diversities in CW were
higher than those in SC when all birds were
included. The diversity in SC was higher than
that in CW when 10 percent of doves were used
in the calculation. Bird species diversities
in CW ranged between 87 percent (AS) and 76
percent (DJF) of maximum when all birds were
used and slightly lower when 10 percent of the
doves were used. In SC BSD percent of maximum
increased in MA, MJJ, and AS when 10 percent
of doves were used in the calculations 65 to
82 percent MJJ, and 60 to 87 percent AS.
Both communities were compared for each
seasonal group using a formula of ecological
overlap (Horn 1966). The salt cedar overlap
with cottonwood-willow was lowest in MA at 50
percent (see Fig. 1) and highest in MJJ at
85 percent. After the peak in MJJ overlap
decreased to 70 percent AS, 65 in ON and to
62 percent DJF.
Conclusion
Historically the study area along the Rio
Grande had large and extensive tracts of
cottonwoods. Past records also demonstrate
that little change occurred until after the
mid 1800's, when the area became settled and
the alluvial floodplain converted to agriculture.
By the turn of the century most of the native
riparian vegetation had been cleared. Today
there are scattered remnants of the native
vegetation, and isolated cottonwoods have been
spared along the edge of fields or canal banks
to offer shade for domestic livestock or workers
during the hot summer months.
Man's activity in the valley brought about
the removal of much of the cottonwood-willow
community. The few remaining trees continue
to flower and fruit, and an occasional seedling
can be found. But intensive grazing by domest~c
livestock quickly eliminates any cottonwood
seedling or sucker that appears. Cottonwoods
are not the only vegetation which is browsed
in the area. Cattle and goats were observed
eating yucca (Yucca spp.), ocotillo (Fouguieria
splendens), prickley pear (Opuntia spp.), and
crown of thorns (Koberlinia spinosa).
Overgrazing has been one factor which has
prevented reproduction of the cottonwood-willow
community in the area. A second factor has
been the reduction of flows and the elimination
of flooding during the time in which cottonwoods
and willow seeds are viable. Cessation of
flooding is the result of increased water use
and the construction of dams upstream. Also
145
With the loss of the cottonwood-willow
community and its replacement by salt cedar,
there was a change in the vertebrate populations.
Salt cedar has a high total bird population in
the breeding season (mostly doves), but during
the winter the densities are very low.
Cottonwood-willow communities not only contain
a higher density of birds than salt cedar but
also support a higher species diversity and
richness. This same pattern was found in
these two communities on the Colorado River
(Anderson and Ohmart 1977).
Several species in the study area have
a high affinity for the cottonwood-willow
association. These are the Brown Creeper
(Certhia familiaris), Common Flicker (Colaptes
auratus), Yellow-bellied Sapsucker (Sphyrapicus
varius). porcupine (Erethizon dorsatum), and
beaver (Castor canadensis). These species
occur in very low densities and will probably
disappear as cottonwoods are extirpated.
Other bird species which will be reduced in
density with the demise of the cottonwood­
willow community are orioles (Icteridae),
Cardi~als (Cardinalis cardinalis), many species
of warblers (Parulidae), Lesser Goldfinches
(Carduelis psaltria), Long-billed Marsh Wrens
(Cistothorus palustris), and some sparrows
(Fringillidae). Mammalian species associated
with the cottonwood-willow community, such as
the cottonrat (Sigmodon hispidus), house mouse
(Mus musculus), and harvest mouse
(R;[throdontomys megalotis) will suffer a
similar fate. Such species as the Black Hawk
(Buteogallus anthracinus), Yellow Warbler
(Dendroica petechia), and arboreal lizard
have already been extirpated in the area.
A 442 km reach of river that was once
dominated by cottonwoods and willows has been
reduced to a few ha of this community. Unless
action is taken to preserve the remnant areas
of cottoriwoods and willows and plant others;
Bieber, R. P. ed. 1935. Exploring southwest
trails, 1846-1854. A. H. Clark,
Glendale, California, Vol. 7, pp. 243­
350.
and protect them from livestock and fire, the
remaining trees will eventually die, leaving
no successors. Plantings of cottonwoods
should be an integral part of any federal. or
state project conducted in this or any other
part of the Rio Grande.
Carothers, S. W. 1977. Importance, preser­
vation and management of riparian
habitat: An overview. In Importance,
Preservation and Management of Riparian
Habitat: A Symposium. USDA Forest
Service Gen. Tech. Report RM-43. p~ 2-4.
At present only a few of the species
that are designated as endangered by the U. S.
Fish and Wildlife Service 1977 live in
cottonwood-willow communities. But as these
habitats continue to be eliminated throughout
the Southwest the species that are restricted
to these habitats may very well be reduced in
number so that they are classified as rare or
endangered.
Emlen, J. T. 1971. Population densities of
birds derived from transect counts.
Auk 88: 323-341.
Johnson et al. (1977) stated that riparian
habitat"s were the most productive and poss:lbly
the most sensitive habitat's inNorth America.
He recommends that they should be managed
accordingly. It is our opinion that Johnson's
recommendations be followed. One of the first
steps would be to initiate efforts to have
cottonwood-willow associations and water
dependent communities in the Southwestern
United States placed on record as being
endangered habi ta t's. These areas should have
the same protection as that accorded endangered
plants or animals.
LITERATURE CITED
Anderson, B. W., R. W. Engel-Wilson, D. Wells,
and R. D. Ohmart. 1977a. Ecological
study of southwestern riparian habitats:
techniques and data applicability. In
Importance, Preservation and Management
of Riparian Habitat: A Symposium. USDA
Forest Service Gen. Tech. Report RM-43.
pp. 146-155.
Anderson, B. W., A. E. Higgins, and R. D.
Ohmart. 1977b. Avian use of salt cedar
communities in the lower Colorado River
Valley. In Importance, Preservation
and Management of Riparian Habitat:
A Symposium. USDA Forest Service Gen.
Tech. Report RM-43. pp. 128-131.
Anderson, B. W., and R. D. Ohmart.1977c.
Vegetative structure and bird use in the
lower Colorado River Valley. In
Importance, Preservation and Management
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Forest Service Gen. Tech. Report RM-43
pp. 23-34.
Emory, W. H. 1859. House Ex. Doc. 135, 34th
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United States and Mexican Boundary
Survey. Washington, D. C. 2 vols.
Engel-Wilson, R. W., and R. D. Ohmart. 1977.
Unpubl. vegetation type map submitted
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Engel-Wilson, R. W., and R. D. Ohmart. 1978.
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vertebrates along the Rio Grande between
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Galvin, J.(ed.)1966. Western America in
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Robinson, T. W. 1965. Introduction, spread
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Wauer, R. H. 1977. Significance of Rio
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